Display screen or portion thereof with a transitional graphical user interface



FIG. 1 is a front view of a display screen or portion thereof with a transitional graphical user interface showing a first image in a sequence of the first embodiment according to claimed design; and,

FIG. 2 is a front view of a second image thereof.

The appearance of the transitional graphical user interface is sequential from FIG. 1 to FIG. 2.

The process or period in which an image transitions to another image forms no part of the claimed design.

The shading in the figures indicate a contrast in appearance.

The dash-dot line shows the display screen or portion thereof and forms no part of the claimed design. The broken line showing of text, numbers and graphic elements forms no part of the claimed design. 

CLAIM The ornamental design for a display screen or portion thereof with a transitional graphical user interface, as shown and described. 